In recent years, with the progress of various portable and cordless electronic appliances, demands for small and light nonaqueous electrolyte secondary batteries having high energy density have been increased, and the development of a positive electrode material for a nonaqueous electrolyte secondary battery has been desired than ever before.
As the material of a positive electrode for a nonaqueous electrolyte secondary battery, LiCoO2, LiNiO2, LiMn2O4 or the like has been used, and especially, LiCoO2 is used in a large quantity from the aspects of safety, capacity and the like. In this material, lithium in the crystal lattice is released into an electrolyte solution as lithium ions with charging, and the lithium ions are reversibly inserted into the crystal lattice from the electrolyte solution with discharging, to exert the functions as a positive electrode active material.
Theoretically, one lithium ion can be released from and inserted into one LiCoO2 lattice. Actually, however, if most of lithium is released or inserted, LiCoO2 is significantly deteriorated causing damage especially to cyclic performance. Therefore, in the present situation, about 0.55 lithium ions are released from and inserted into one LiCoO2, and at this time, the capacity of only about 150 mAh is used for 1 g of LiCoO2.
Although the expansion of the capacity can be expected by releasing and inserting larger quantities of lithium ions, if more lithium ions than the present quantities are released and inserted, there are problems that intense deterioration of LiCoO2 occurs and sufficient cyclic performance cannot be secured by the phase transition of the LiCoO2 crystal lattice, accompanying damage of particles and crystal lattice, or the elution of cobalt ions from the crystal lattice.
Although there has been an attempt to improve the cyclic durability at 4.5 V by doping 5% by weight of zirconium into LiCoO2, the initial capacity lowers significantly, and cyclic durability is also not satisfactory (refer to Non-Patent Document 1 described below).
[Non-Patent Document 1]: Z. Chen, J. R. Dahn, 11th International Meeting of Lithium Battery Jun. 23-28, 2002, Monterey, USA, Abstract No. 266